Nanosized transition metal oxides, because of their high surface areas, are promising candidates for a vast range of applications including negative electrode materials for lithium-ion batteries, photoelectrochemical cells, electrochromic display, and sensors. Among various metal oxides, tungsten oxide has found useful application in electrochromic devices, semiconductor gas sensors, and photocatalysis. Especially, one-dimensional tungsten oxides have been of special interest due to their lower dimensionality and superior properties. Recently, tungsten oxide nanowires or nanorods were synthesized by heat treatment of tungsten metal and wet chemical reaction.
In this study, various tungsten oxide nanostructures were synthesized via simple solvothermal method with different kinds of solvents. Firstly, the tungsten hexachloride $(WCl_6)$ was dissolved in ethanol, 1-propanol, 2-propanol, and 1-butanol. Then the starting solution was mixed with various solvents, and the solvothermal reaction was conducted at 125 ~ 200℃ for 10 hours. The numerous morphologies of tungsten oxide could be obtained, i.e., square nanosheets, hexahonal platelets, nanorods, nanowires, nanobundles and nanourchins. All the phase of 1-dimensional tungsten oxide was $W_{18}O_{49}$, and the diameter of the building unit (nanowire) is about 7 nm. The dielectric constant of solvents influenced on the agglomeration of particles, and the orientation of assembled particles could be explained by molecular structure of solvents.
$W_{18}O_{49}$ nanobundle film was fabricated by Langmuir-Blodgett (LB) method. The two types of nanobundles were classified by long nanobundles ( ~ 800 nm in length) and short nanobundles ( ~ 400 nm in length). Locally arranged domains of the long nanobundles form the LB films, but it is difficult to align perfectly owing to inter-nanobundles interaction and dispersion problem. Since short nanobundles were separated into smaller nanobundles by ultrasonification, it is difficult to deposit uniformly and obtain high surface pressure. Well deposited LB film of long nanobundles showed clear UV edge compared to the suspension.
Electrochromic (EC) property of tungsten oxide nanobundle LB films was characterized by electrochemical potential cycling tests. As the heat treatment temperature increased, current density was lowered and the difference of transmittance between colored state and bleached state ($\Delta T$) also decreased. The deposition condition of LB films has influenced on the EC property. The transmittance difference ($\Delta T$) of long nanobundles was 23 ~ 34 %, and that of short nanobundles was 14 ~ 22 %. It seemed that the surface pressures have influence on EC property, but good deposition state with proper surface pressure is considered as more important effect rather than surface pressure only. Although more profound experiments and discussion is needed to investigate the relation with tungsten oxide nanostructures and EC property, but it is meaningful work that firstly shown as the low dimensional tungsten oxide exhibits good electrochromic property.
